Waveform synthesizing systems and methods are widely used in electronic systems such as communication systems. A synthesizing apparatus synthesizes from an input waveform, an output waveform in a load using a DC power supply. When the input waveform is an AC input waveform, the synthesizing apparatus may be regarded as an amplifier. When the input waveform is a DC input waveform, the synthesizing apparatus may be regarded as a DC-to-AC power converter. Synthesizing apparatus are widely used as power amplifiers in audio-video systems, instrumentation and in radio frequency communication systems. In radio frequency communication systems, synthesizing apparatus are widely used in transmitters of base stations and transmitters of mobile radiotelephones.
A major consideration in the design of power amplifiers is the efficiency thereof. High efficiency is generally desirable so as to reduce the amount of power that is dissipated as heat. Moreover, in many applications, such as in satellites and portable radiotelephones, the amount of power that is available may be limited. An increase in efficiency in the power amplifier is therefore important, in order to allow an increase the operational time or capacity for the satellite or portable radiotelephone.
Conventional DC-to-AC power converters include square wave inverters, modified sinewave inverters and true sinewave inverters. Square wave inverters convert DC-to-AC power, but their square-wave output signal waveform may contain large amounts of odd harmonic energy. Certain electronic devices do not operate efficiently when large harmonic content is present in the output waveform. For example, radio or audio interference may occur when attempting to power a radio or TV set from such an inverter. An additional problem with square wave inverters is that the peak and rms values of the waveform generally do not have the same ratio of .sqroot.2 as in a conventional sinewave supply. Certain loads, such as lamps, only require that the RMS value of a power source should be correct. However, other loads including transformer-rectifier arrangements, may operate correctly only if the peak voltage level is correct. Therefore, all loads may not operate correctly from a square waveform.
The above problems may be partly overcome by using a modified sinewave inverter. A modified sinewave inverter is generally a modified square wave inverter, modified to produce a 3-level output waveform of levels+Vpeak, 0, -Vpeak, 0 . . . in repetitive sequence. Introduction of the 0-level for a properly chosen proportion of the time allows the waveform to have the same peak-to-rms ratio as a sinewave, thus extending the range of apparatus designed for sinewave operation that can be correctly powered by the inverter. However, the odd harmonic content of the waveform may increase in this case, and loads such as motors that are less efficient when large harmonic content is present may still not function efficiently. Thus, there was still a need in the prior art for "true sinewave" inverters.
A true sinewave inverter can be made using a class-B linear amplifier to amplify a sinewave signal to a high power level. However, such an amplifier may achieve a maximum DC-to-AC power conversion efficiency of .pi./4 or 78.5% even using ideal components. Another prior art means to produce a true sinewave inverter comprises using square-wave switching devices in conjunction with inductor-capacitor filtering to remove harmonics, thereby converting the square switching waveform to a sinusoidal output waveform. However, inverters based on filtering may require very large filtering components and may suffer from poor voltage regulation when loaded by different amounts.
Other prior art true sinewave inverters have been reported in which several square-wave inverters operating at, for example the line frequency, 3.times. the line frequency, 5.times. the line frequency, etc. have their outputs combined such that the odd harmonic content cancels. Such converters can achieve high efficiency but may be limited in the accuracy of the waveform. They are also generally adapted to convert DC-to-AC power only for a particular waveform, and not for a general waveform such as an audio or radio signal.
It is also known to use a digital-to-analog (D/A) converter as a waveform synthesizer, where the input waveform is a digital waveform. A well known type of D/A converter is a weighted-resistor D/A converter. The weighted resistor D/A converter uses resistor values that are weighted, so that their resistances are inversely proportional to the numerical significance of the corresponding binary digit. The resistors are coupled to a load by a corresponding plurality of switches. The switches may be field effect transistors or complementary bipolar transistors. See pages 494-516 of "Digital Integrated Electronics" by Taub and Schilling, 1977.
Notwithstanding all of the above approaches, there continues to be a need for waveform synthesizers that can synthesize waveforms at high efficiency.